1. Field of the Invention
This invention relates to improved microporous waterproof and moisture vapor permeable structures, and more particularly it relates to such structures which primarily comprise known microporous polytetrafluoroethylene (PTFE) or microporous interpenetrating matrices of PTFE and polydiorganosiloxane, and which have been treated in accordance with the process of this invention with either a curable or a non-curable hydrophobic silicone composition, or mixtures thereof, to produce microporous waterproof and moisture vapor permeable structures having improved physical characteristics and having resistance to surfactant activity.
2. Background Information
In U.S. Pat. No. 4,187,390 to Gore, there is disclosed a process of producing microporous PTFE membranes which are waterproof yet moisture vapor permeable. Such membranes are characterized by a microstructure consisting of nodes interconnected by fibrils.
In our copending application (Ser. No. 000,389 filed Jan. 5, 1987, U.S. Pat. No. 4,945,125, the disclosure of which is hereby incorporated by reference) there is disclosed a process of producing a fibrillated (having nodes interconnected by fibrils) semi-interpenetrating polymer network (SIN) of PTFE and silicone, and shaped products thereof.
In the production of microporous PTFE membranes according to the Gore ('390) process, or the production of microporous PTFE/silicone SIN membranes according to our ('389) process, linearly oriented extrudate of a biaxial fibrillation process is further biaxially oriented by use of equipment such as tenter frames or the like. The microporous membranes thus produced are then normally heated to above 327.degree. C. and subsequently cooled to effect sintering of the PTFE, the resulting films having waterproof and moisture vapor permeable characteristics.
In U.S. Pat. No. 3,325,434 to Tully, there is disclosed an extrudable PTFE composition containing 0.6% to 12.5% by weight of uncured silicone rubber, said silicone principally serving to fill the voids created as the organic processing aid is volatilized after wire extrusion, thereby rendering a structure which has outstanding electrical properties.
In U.S. Pat. No. 4,194,041 to Gore, there is disclosed a layered article comprising a microporous membrane of PTFE having one surface in contact with a layer of hydrophilic materials so as to maintain moisture vapor diffusion while preventing contamination of the PTFE membrane from surfactants or the like. Articles produced according to this process have been found to have substantially less moisture vapor permeability in comparison to conventional microporous PTFE membranes.
In U.S. Pat. No. 4,613,544 to Burleigh, there is disclosed a unitary sheet of microporous polymeric matrix having continuous pores, said pores being sufficiently filled with a hydrophilic material so that moisture vapor is enabled to permeate the structure only by molecular diffusion.
In U.S. Pat. No. 4,500,688 to Arkles there is disclosed a melt processable pseudo-interpenetrating polymer network of silicone in thermoplastic matrices.
In U.S. Pat. No. 4,764,560 to Mitchell there is disclosed polymeric structures having interpenetrating matrices in cured form, comprising a PTFE network and a polydiorganosiloxane network.
The disclosures of each of the foregoing patents are incorporated herein by reference.